Method of producing a basic powder for permanent magnets

ABSTRACT

METHOD OF PRODUCING A BASIC POWDER FOR PERMANENT MAGNETS. THE POWDER IS FORMED BY THE COMPOUND M5R, WHEREIN M=CO, COMBINED, AS THE CASE MAY BE, WITH FE, NI, CU AND R IS AN ELEMENT OF THE RARE EARTHS INCLUDING TH. THE ESSENTIAL FEATURE OF THE METHOD IS THAT THE POWDER IS OBTAINED BY GRINDING IN A HYDROGEN ATMOSPHERE. THE ADVANTAGE IS THAT IMPROVED COMBIANTIONS OF ORIENTABILITY AND COERCIVE FORCE ARE OBTAINED IN THAT (A) THE COERCVE FORCE AS A FUNCTION OF THE GRINDING PERIOD INCREASES VERY RAPIDLY; AND (B) THE MAXIMUM OBTAINABLE COERCIVE FORCE IS HIGHER. THE PERMANENT MAGNETS MANUFACTURED BY MEANS OF THIS BASIC POWDER ARE SUPERIOR TO THOSE OBTAINABLE BY THE KNOWN GRINDING METHODS WITH RESPECT TO THE COMBINATION OF (BH)MAXAND HC.

, A 8- 1972 H. c. Do kERsLoo'r ETAL 3,681,151

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INVENTORS HENDRIK C.D0NKERSL0OT JOHANNES H.N.VAN VUCHT 7 Aug. 1, 1972 I Q DQNKERSLQOT ETAL 3,681,151

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INVENTORS' HENDRIK C.DONKERSLO0T JOHANNES H.N.VAN VUCHT AGENT United States Patent Ofice Patented Aug. 1, 1972 3,681,151 METHOD OF PRODUCING A BASIC POWDER FOR PERMANENT MAGNETS Hendrik Cornelis Donkersloot and Johannes Hendrikus Nicolaas van Vucht, Emmasingel, Netherlands, assignors to U.S. Philips Corporation, New York, N.Y.

Filed Mar. 23, 1971, Ser. No. 127,144 Claims priority, application Netherlands, Apr. 18, 1970, 7005646 Int. Cl. H01f 1/06 U.S. Cl. 148-105 1 Claim ABSTRACT OF THE DISCLOSURE Method of producing a basic powder for permanent magnets. The powder is formed by the compound M R, wherein M=Co, combined, as the case may be, with Fe, Ni, Cu and R is an element of the rare earths including Th. The essential feature of the method is that the powder is obtained by grinding in a hydrogen atmosphere. The advantage is that improved combinations of orientability and coercive force are obtained in that (a) the coercive force as a function of the grinding period increases very rapidly; and (b) the maximum obtainable coercive force is higher.

The permanent magnets manufactured by means of this basic powder are superior to those obtainable by the known grinding methods with respect to the combination of (BH),,,,, and H The invention relates to a method of manufacturing a basic powder forming the building stone for permanentmagnetisable bodies essential for the permanent-magnetic properties, said bodies consisting of a compound having a hexagonal crystal structure and having a range of existence 'which is integral with that of the compound M R in the M-R-system, wherein M represents Co or a combination of Co with one or more of the elements Fe, Ni and Cu and R represents one or more of the elements of the rare earths and/or Th, said powder being obtained by pulverizing castings of the compound.

The elements of the rare earths are considered herein to include also the element Y.

Dutch patent application No. 6807894 laid out for public inspection discloses such a method.

The basic powder to be manufactured has to satisfy inter alia the requirement that both the coercive force H and the orientability in a magnetic field should be optimal. A measure for this orientability and moreover, for the squareness of the magnetic hysterisis loop, is the ratio a /tr wherein a, is the saturation magnetisation and a, the remanent magnetisation.

From said Dutch patent application No. 6807894 it is known that when castings of the alloys concerned are pulverized in a first instance both the H and the (T /0' increase according as grinding is prolonged. From said publication it furthermore appears that both H and o /o' exhibit maxima as a function of the grinding period. See in particular FIG. 4 of said application and FIG. I of the article of Strnat et al. in Journal of Applied Physics 1968, pages 1263 to 1265. These maxima, however, do

not occur after the same grinding period: a maximum r,/a, is attained earlier than a maximum H Consequently, when an optimum orientability of the powder is required, a coercive force not yet having attained the maximum will have to be accepted, whereas with a requirement of maximum coercive force the orientability can no longer be the optimum. Since the orientability of the powder becomes directly manifest in the maximum energy product (BH) of a permanent magnet made therefrom, the foregoing means that such a permanent magnet cannot simultaneously exhibit a maximum (BH) and a maximum H This is a disadvantage.

This disadvantage is even more conspicuous 'when such a permanent magnet is a so-called compressed magnet i.e. a magnet made from compresed powder. When a basic powder having a given coercive force is compressed into a magnet-body, the coercive force of said magnet-body is always lower than that of the basic powder. This may involve that with a maximum (BHL the coercive force becomes too low for certain uses.

By the method according to the invention said disadvantages are considerably mitigated and sometimes even completely obviated. Said method is characterized in that the castings are pulverized in a hydrogen gas atmosphere.

As the case may be the castings are first crushed, for example, into particles having a maximum diameter of 300 nn, which particles are subsequently pulverized in accordance with the invention.

In principle the advantages of the invention are of two kinds:

(1) As a function of the grinding period the coercive force of the powder increases much more rapidly than in the methods hitherto known and disclosed, for example, in Dutch patent application 6807894. Whilst the increase in orientability with the grinding period generally does not vary. This means that the coercive force attains its maximum value earlier-i.e. after a shorter grinding period-and hence that if grinding is prolonged until the orientability is at its maximum the associated coercive force of the powder is higher than the value attainable by the known methods.

(2) The maximum attainable coercive force is considerably higher than that of a basic powder of the same composition, but pulverized by one of the known methods.

By means of the graph of FIG. 1 the aforesaid advantages will be illustrated: the variation of the coercive force H and that of the orientability o' /o' of the powder are shown as a function of the grinding period. H is the coercive force measured on powder ground in argon, H is the coercive force measured on powder ground by the method in accordance with the invention. The 0 /1 curve is found to coincide substantially with that measured on powder ground in an inert atmosphere, the first powder being ground in hydrogen. A measurement of the H and (r /0' of the powder after, for example, a grinding period will emphasize the advantage of the method in accordance 'with the invention, the combination (H =A; a,/ ,=B) is less advantageous than the combination (H,,=C; o',/o' =B).

For a few compounds of M and R in accordance with the invention the following table indicates the eifect of the method in accordance with the invention on the magnitude of the coercive force and the orientability of the powder; the values of the coercive force and the orientability are measured on powders pulversized in an argon atmosphere at different hydrogen gas pressures P The basic material was invariably a sieved fraction of crushed castings of the composition concerned, the diameter of the particles being at the most 300 pm. Pulverization was carried out in a vibration apparatus in which the grits were subjected to vibrations for 30 minutes.

Ce(MM) represents Ce-rich mixed metal (Mischmetall) of the following composition: 90% by weight of Ce, 7% by weight of La, remainder further elements of the rare earths.

TABLE M-R compound (atrn.) H, (00.) lo,

l O 10, 000 O. 95

GdCo as ,000 0.95 1. 6 14, 000 0. 95

SmCOm 0. 4 12, 400 0. 97 0. 8 19, 400 0. 98

a: as as Ce(MM)C0 0. 4 1, 200 0. 61 0. 8 1,800 0. 67

l Argon.

The graphs of FIGS. 2 and 3 illustrate to what extent the coercive force of the compounds mentioned in the table depends upon the hydrogen gas pressure during pulverization. It is found that already at very low hydrogen gas pressures an appreciable increase in coercive force occurs, whilst dH, dP

decreases at higher hydrogen gas pressures.

From the table it appears in accordance with the foregoing that the a /o' as a function of the hydrogen gas pressure during pulverization remains in general practically constant. An exception is formed by the compound SmCo whose er /0' apart from H increases with an increasing hydrogen gas pressure. This means that for this compound-and in general for compounds in which M=Co and R=Sm lying within the range of existence referred to in the preamble-the method according to the invention provides an additional advantage.

For the compound SmCo the graph of FIG. 4 illustrates the er /d as a function of the hydrogen gas pressure.

Concise description of the grinding process carried out on the compounds on which the H and er /0' values indicated in the table have been measured.

A melt of the compound obtained in known manner by an arc was crushed in a steel impact morsar. A fraction of a diameter of less than 300 ,um. was sieved from the grits and transferred to a grinding tub of hardened stamp steel; total weight of the powder about 2 g. The grinding tub had a capacity of about cm. and contained 250 steel balls (diameter 3 mm.). The tub was closed hermetically as well as the supply ducts between the interior of the tub and an argon or hydrogen gas cylinder respectively were exhausted vacuum (10- mm. Hg). By means of a regulating cock with a reducing valve the pressure in the tub could be adjusted and be kept constant during the vibration process. The grinding tub was then transferred to a vibration apparatus and subjected to about 1400 vibrations a minute for 30 minutes. After this vibration process the average granular size was found to be less than 1 am.

After the vibration process and after the supply of argon and hydrogen gas respectively had been cut, the space inside the tub above the powder and the balls was again exhausted. Finally the powder was removed from the tub and in known manner the H and u /tr, were measured.

The hydrogen gas employed contained less than 10 p.p.m. of oxygen and/or water vapour.

The invention also relates to basic powders for the manufacture of permanent magnets by means of the method in accordance with the invention and to permanentmagnetisable bodies, magnetized or non-magnetized and composed from fine particles whose constituent essential for the permanent-magnetic properties is formed by such a basic powder. The permanent-magnetisable bodies may be formed from compressed powder or sintered powder.

What is claimed is:

1. A method of manufacturing a basic powder essential for the permanent-magnetic properties for permanentmagnetisable bodies consisting of a compound having a hexagonal crystal structure, the composition of which corresponds with that of the compound M R in the M-R system, where M is an element selected from the group consisting of Co and Co with at least one of the elements Fe, Ni and Cu and R is an element selected from the group consisting of the rare earth and Th, comprising the step of pulversing castings of the compound.

References Cited UNITED STATES PATENTS 7/1967 Griest, Jr., et al 148105 8/1969 Becker 148-105 US. Cl. X.R. 

